1. Field of the Invention
The present invention relates to a diffractive-optical element, a scanning optical system, an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
An optical scanning device guides beams emitted from a light source to an optical deflector by a first optical system, deflects the beams by the optical deflector, forms the deflected beams on a surface to be scanned as an optical spot by a second optical system, and optically scans the surface. Such an optical scanning device is widely known in relation to image forming apparatuses such as digital copiers, facsimile machines, optical printers, and optical plotters.
A semiconductor laser and a light-emitting diode (LED) used generally as light sources for such an image forming apparatus are monochromatic light sources. However, their light-emitting wavelengths differ by the type of the semiconductor laser and the LED. A photosensitive body optically scanned by the optical scanning device has different spectral characteristics of light sensitivity according to its type. In general, a light source that has a light-emitting wavelength that is suitable to photosensitive characteristics of the photosensitive body is selected.
In the optical scanning device, a scanning optical system that focuses the light emitted from the light source on the photosensitive body as an optical spot includes a lens that is a refractive optical system. The refractive index of a lens material differs by its wavelength because the material has a unique dispersion. Therefore, in related art, such a scanning optical system is designed based on the light-emitting wavelength of the light source to be used. An optical element used for the scanning optical system has no compatibility with light source wavelengths.
It is known in Japanese Patent Application Laid-Open No. 2006-085487 that a fluctuation caused by temperature change at a point of focus of a scanning optical system can be reduced by combining with another lens, using a diffractive-optical element. The diffractive-optical element has diffractive functions as an optical element used for the scanning optical system. However, this system does not improve the compatibility of the diffractive-optical element with light source wavelengths.
The optical scanning device has been widely known in relation to image forming apparatuses such as optical printers, digital copiers, and optical plotters. In recent years, such an optical scanning device that not only achieves a reduction in costs, but also is robust against environmental fluctuations and able to form a high-resolution image has been demanded.
To effectively reduce the costs of the optical scanning device, various types of lenses used for the optical scanning device are formed with a resin material. The resin material is light in weight and can be formed at a low cost. Special surface shapes, an aspheric surface in particular, can be formed easily. Adopting a special surface to the resin lens can improve optical characteristics, and reduce the number of lenses included in the optical system.
On the other hand, as known widely, the shape and the refractive index of the resin lens change due to environmental fluctuations, particularly, changes in temperature. Therefore, the optical characteristics, particularly power, change from a design value. This leads to a problem that a beam spot diameter that is the diameter of the optical spot on a surface to be scanned fluctuates due to the environmental fluctuations.
The power fluctuations of the resin lens due to temperature changes occur to a positive lens and a negative lens reversely with each other. A method of canceling out the fluctuation of the optical characteristics caused by environmental fluctuations that occurs at the positive and the negative resin lenses in the optical system of the optical scanning device, including the positive and the negative lenses, are well known.
The semiconductor laser commonly used as the light source of the optical scanning device has a nature that the emitting light wavelength shifts towards a long-wavelength side with temperature rise (the wavelength fluctuation caused by temperature change). The wavelength fluctuation in the light source imposes characteristic fluctuations caused by color aberration of the optical system used for the optical scanning device. Accordingly, this characteristic fluctuation also causes the fluctuation of the beam spot diameter.
The optical scanning device disclosed in Japanese Patent Application Laid-Open No. 2002-287062 that stabilizes the optical characteristics by adopting a diffraction surface is known. The fluctuation of the optical characteristics of the optical element caused by temperature change and the wavelength fluctuation in the light source are considered in the Japanese Patent Application Laid-Open No. 2002-287062.
Using the diffraction surface is effective for stabilizing the optical characteristics of the optical scanning device. Forming the diffraction surface generally requires a micro-fabrication technique with extremely high precision. For example, a diffraction surface that has a power equivalent to a spherical lens may be formed by grooves of concentric circles. However, intervals of the grooves rapidly narrow with an increase of a radius of the concentric circles. Accordingly, it is not necessarily easy to form high precision grooves, and this has become a barrier for forming the diffraction surface at a low cost.